The invention relates to apparatus for detecting the oscillation amplitude of an oscillating object and in particular, the oscillation amplitude of the capillary tip of an ultrasonic transducer for ultrasonic welding.
During the semiconductor packaging process a semiconductor die (or chip) is bonded to a metal leadframe. This is commonly known as die attachment. Conductive wire is then bonded between electrical contact pads on the die and electrical contacts on the leadframe by a piece of equipment commonly known as a wire bonder. The wire bonder bonds the conductive wire to the die and the leadframe by an ultrasonic welding process, which uses an ultrasonic wave transducer. The ultrasonic wave transducer has a capillary working tip mounted on it and the conductive wire passes through a through bore in the capillary to the capillary tip. It is the tip, which applies the ultrasonic vibration from the transducer to the conductive wire to form the bond. The transducer generates longitudinal vibration of the capillary tip, which bonds the wire onto the die pad or the leadframe.
The oscillation amplitude of the capillary tip has been identified as one of the critical parameters necessary to achieve consistent bonding results. Due to the very small size of the capillary tip and the complex vibration pattern, it is difficult to accurately measure the vibration amplitude of the capillary tip in both free (unloaded) vibration mode and loaded vibration mode. A further complication is that different capillaries used in different transducers have different vibration patterns. A large number of attempts have been made in recent years to develop systems to measure the oscillation amplitude accurately.
However, these systems either can not perform real-time measurement or involve a complex series of operations in a controlled environment. With some of the systems it is even necessary to switch off the wire bonder during the measurement process.
For example, U.S. Pat. No. 5,199,630 measures the transducer""s vibration amplitude by using an opto-electronic receiver and a corresponding electronic controller. To perform the measurement, the apparatus must be re-calibrated every time and thus cannot perform real-time measurement. To measure the transducer""s vibration, the apparatus must be fixed to the bonding area of the wire bonder. The apparatus needs to be removed from the bonder after measurement for normal operation of the wire bonder. Hence, the apparatus can not be used to measure the oscillation amplitude during an actual wire bonding operation. Therefore, this apparatus is not practical to conduct frequent amplitude measurements.
This apparatus is also sensitive to the ambient temperature during the measurement process as it measures only the optical power variation due to the vibration of the transducer. Therefore, this apparatus is not convenient to use in an industrial environment where the temperatures in the vicinity of the capillary tip can be high due to the bonding operation.
Furthermore, the apparatus disclosed in U.S. Pat. No. 5,199,630 measures the oscillation amplitude of the ultrasonic transducer that holds the capillary tip, not the vibration amplitude of the actual capillary tip. When one capillary is replaced with a new capillary, for example due to wear of the capillary or a different capillary is needed to bond a new device, the actual vibration of the capillary may be different.
Therefore, the measurement of the oscillation amplitude of the transducer cannot be used to precisely monitor the quality of the bond, as the oscillation amplitude measured does not accurately reflect the oscillation amplitude of the capillary tip.
In accordance with a first aspect of the present invention, apparatus for detecting the oscillation amplitude of an oscillating object comprises an optical radiation source; a detector comprising first and second optical radiation sensing areas adjacent each other, the detector and the optical radiation source adapted to be located opposite each other with the oscillating object located between the source and the detector so that the object blocks a portion of the sensing areas from receiving optical radiation from the source; and a processor coupled to the detector to receive first and second output signals representing the magnitude of optical radiation sensed by the first and second optical radiation sensing areas, respectively; the processor processing the first and second output signals to obtain an indication of the amplitude of oscillation of the object.
In accordance with a second aspect of the present invention, a method of detecting the oscillation amplitude of an oscillating object comprises positioning an optical radiation source and an optical radiation detector on opposite sides of the object, the detector comprising first and second optical radiation sensing areas; illuminating the object with optical radiation from the source and processing first and second output signals from the first and the second optical radiation sensing areas to determine the oscillation amplitude of the object.
The term xe2x80x9coptical radiationxe2x80x9d as used herein covers electromagnetic radiation in the visible, ultraviolet and infrared regions of the electromagnetic spectrum.
An advantage of the invention is that it permits the amplitude of oscillation of a capillary tip of an ultrasonic bonder to be measured without influencing the vibration of the capillary tip. This enables real-time measurement of the vibration amplitude of the capillary tip of a wire bonder and enables the transducer to be calibrated to produce consistent vibration amplitude of the capillary tip and to thereby improve the bond quality.
Preferably, the oscillating, object is a tip of an ultrasonic transducer in an ultrasonic welding machine. Typically, where the ultrasonic welding machine is a wire bonder, the tip is a capillary tip.
Typically, the processor may generate an output oscillation signal, which can be applied to the oscillating object to modify the oscillation amplitude of the object in response to the oscillation amplitude detected by the processor. This has the advantage that as well as measuring the oscillation amplitude, the apparatus may also control the oscillation amplitude in response to the measured oscillation amplitude.
Preferably, the output oscillation signal is input to a control device that controls oscillation of the object. Typically, where the oscillating object is a tip of an ultrasonic transducer, the control device comprises an ultrasonic wave controller.
Typically, the control device compares the oscillation amplitude with a reference oscillation amplitude and controls the oscillation of the object so that the object oscillates at substantially the reference oscillation amplitude. Preferably, the control device controls the oscillation amplitude in real time.
Typically, the optical radiation source comprises a collimating device to collimate the optical radiation exiting the source.
Preferably, the width of each of the first and second optical radiation sensing areas is greater than the sum of half the width of the oscillating object and the amplitude of oscillation of the object. Typically, the amplitude of oscillation is less than the width of the oscillating object.
In one example of the invention, the first and second optical radiation sensing areas are directed towards the optical radiation source. Typically, the first and second optical radiation sensing areas are adjacent each other. The optical radiation sensing areas may be coplanar. Typically, the spacing between the first and second radiation sensing areas is not greater than 10% of the width of the oscillating object. Preferably, the spacing is less than 10% and is kept to minimum.
In an alternative example of the invention, the first and second optical radiation sensing areas are not directed towards the optical radiation source and the detector further comprises an optical device to direct the optical radiation onto the first and second sensing areas.
Preferably, the processor generates an indication of the oscillation amplitude by comparing the sum of the first and second output signals with the difference between the first and second output signals.
Typically, the first and second optical radiation sensing areas each comprise a photodiode.